It is conventional practice in the oil and gas industry to cement casing in well bores. The cement is placed in the annulus between the casing and the well bore, and is intended to secure and support the casing in the well bore and to isolate various formations from one another by preventing migration of formation fluids up and down the well bore. The cement is generally pumped down the casing interior, out into the well bore annulus, and back up toward the surface to the desired level. Secondary and tertiary cementing operations are very often utilized to isolate producing horizons from migrating water or other well fluids from other levels. Such cementing thus occurs along the length of the casing pipe string after primary cementing of the foot of the casing string at the desired level. Accurate location of the higher level cement columns or cement plugs and use of only the appropriate amounts of cement slurry for each of such intervening locations is highly desirable. Likewise, the ability to remove any cementing apparatus which might interfere with further down-hole operations with a drill stem is necessary for further treatment and development of the well.
Secondary and tertiary stage cementing serves several desirable purposes. By lowering hydrostatic weights and pump pressures, it reduces the potential for formation damage. It cuts material cost when cementing widely separated intervals. It minimizes cement contamination. Stage cementing also is useful in placing retarded slurries in hot bottom-hole temperatures, when such cement might fail to set if allowed to rise to a cooler zone.
Cementing tools of the prior art generally have required separate mechanisms to perform the required steps of opening passages for the cement being charged to be deposited in the annulus of the casing and into the surrounding formation and thereafter securely closing such passages until the cement charge has hardened and cured. Often additional collars and baffles have been required to prevent such cement from backing up in the interior of the casing when positive hydrostatic pressure from the surface is released. All these added items of equipment have served to multiply the difficulties, increase the costs and afford more opportunities for failure of the desired cement plugs.